In lithography, lithographic exposure tools for transferring a fine circuit pattern onto a wafer to produce an integrated circuit have been extensively used. With the trend toward higher degrees of integration, higher speeds, and higher functions in integrated circuits, the integrated circuits are becoming finer and the lithographic exposure tools are required to have a large focal depth and form a high-resolution circuit pattern image on a wafer surface. The wavelengths of exposure light sources are becoming shorter. ArF excimer lasers (wavelength, 193 nm) have come to be used as exposure light sources in place of the g-line (wavelength, 436 nm), i-line (wavelength, 365 nm), and KrF excimer lasers (wavelength, 248 nm) heretofore in use. Furthermore, use of an F2 laser (wavelength, 157 nm) as an exposure light source for conforming to next-generation integrated circuits having a line width of 100 nm or smaller is thought to be promising. However, the generations which can be covered by this light source are regarded as being limited to ones with line widths down to 70 nm.
Under such technological trends, a lithographic technique employing EUV light as a next-generation exposure light source is thought to be applicable to generations of 45 nm and finer and is attracting attention. EUV light means a light having a wavelength in the soft X-ray region or vacuum ultraviolet region. Specifically, it is a light having a wavelength of about 0.2-100 nm. At present, use of a lithographic light source of 13.5 nm is being investigated. The exposure principal in this EUV lithography (hereinafter abbreviated as “EUVL”) is equal to that in the conventional lithography in that a mask pattern is transferred with an optical projection system. However, since there is no material which transmits light in the EUV light energy region, a refractive optical system cannot be used and a reflective optical system should be used (see patent document 1).
The mask for use in EUVL is basically constituted of (1) a glass substrate, (2) a reflecting multilayered film formed on the glass substrate, and (3) an absorber layer formed on the reflecting multilayered film. As the reflecting multilayered film is used a film having a structure formed by periodically superposing, in a nanometer-order thickness, materials differing in refractive index at the wavelength of the exposure light. Known typical materials are molybdenum and silicon. Tantalum and chromium are being investigated as materials for the absorber layer. The glass substrate is required to be a material having a low coefficient of thermal expansion so as not to be distorted even upon EUV irradiation. Use of a glass having a low coefficient of thermal expansion or a crystallized glass having a low coefficient of thermal expansion is being investigated. In this description, glasses having a low coefficient of thermal expansion and crystallized glasses having a low coefficient of thermal expansion are hereinafter referred to inclusively as “low-expansion glasses” or “ultralow-expansion glasses”.
The low-expansion glass or ultralow-expansion glass most widely used in EUVL masks is quartz glass which comprises SiO2 as the main component and contains TiO2, SnO2, or ZrO2 as a dopant so as to have a reduced coefficient of thermal expansion.
A glass substrate is produced by processing such a glass or crystallized-glass material with high accuracy and cleaning it. A glass substrate is processed generally in the following manner. A surface of the glass substrate is pre-polished at a relatively high processing rate until the surface comes to have a given degree of flatness and a given surface roughness. Thereafter, the glass substrate surface is finished by a method having higher processing accuracy or under processing conditions bringing about higher processing accuracy so as to result in a desired degree of flatness and surface roughness.
Patent Document 1: JP-T-2003-505891